Computer Memory Devices

In a computer, memory is one or more sets of chips that store data and/or program instructions, either temporarily or permanently. Memory is a critical processing component in any computer.
Personal computers use several different types of memory, but the two most important are called random access memory (RAM) and read-only memory (ROM). These two types of memory work in very different ways and perform distinct functions.

Random Access Memory

The most common type of memory is called random access memory (RAM). As a result, the term memory is typically used to mean RAM. RAM is like an electronic scratch pad inside the computer. RAM holds data and program instructions while the CPU works with them. When a program is launched, it is loaded into and run from memory. As the program needs data, it is loaded into memory for fast access. As new data is entered into the computer, it is also stored in memory—but only temporarily. Data is both written to and read from this memory.
Fig. 1: Random Access Memory (RAM)
(Because of this, RAM is also sometimes called read/write memory.) Like many computer components, RAM is made up of a set of chips mounted on a small circuit board (see Fig. 1).
RAM is volatile, meaning that it loses its contents when the computer is shut off or if there is a power failure. Therefore, RAM needs a constant supply of power to hold its data. For this reason, you should save your data files to a storage device frequently, to avoid losing them in a power failure. (You will learn more about storage later in this chapter.)
RAM has a tremendous impact on the speed and power of a computer. Generally, the more RAM a computer has, the mote it can do and the faster it can perform certain tasks. The most common measurement unit for describing a computers memory is the byte—the amount of memory it takes to store a single character such as a letter of the alphabet or a numeral. When referring to a computer's memory, the numbers are often so large that it is helpful to use terms such as kilobyte (KB), megabyte (MB), gigabyte (GB), and terabyte (TB) to describe the values (see Table 1).
Table. 1: Units of Measure for Computer Memory and Storage
UnitAbbreviationApproximate Value (bytes)Actual Value (bytes)
MegabyteMB(1 million) 1,048,576
GigabyteGB(1 billion) 1,073,741,824
TerabyteTB(1 trillion)1,099,511,627,776
Today’s personal computers generally have at least 2 billion bytes (2 GB) of random access memory. Many newer systems feature 4 GB or more.

Read-Only Memory

Unlike RAM,
read-only memory (ROM) permanently stores its data, even when the computer is shut off. ROM is called nonvolatile memory because it never loses its contents.
ROM holds instructions that the computer needs to operate. Whenever the computer's power is turned on, it checks ROM for directions that help it start up, and for information about its hardware devices.

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